The generator stator core is the largest component in the train of a turbine generator set. The stator cores are generally manufactured from thousands of laminations of relatively thin steel plates which are stacked, pressed and clamped together into the large cylindrical form of the stator core. Typically, the stator core is assembled from the steel plates directly at the final installation site. However, the large size of the stator core and the need for proper clamping results in stator core manufacturing difficulties, including generous floor space and high crane requirements. U.S. Pat. No. 5,875,540 by Sargeant, which is incorporated herein by reference, overcame some of the problems with the prior art by first assembling a number of laminations into a distinct set, referred to as a donut, and then stacking these donuts to form a stator core. This saved great amounts of time over assembling the laminations individually, and produced a stator core with less flaws. When the individual laminations, or the set of laminations in a donut, are installed into a core, they engage what are referred to as keybars. Keybars are essentially rods that run the internal length of the stator core and provide a hook-in spot for the laminations. Laminations are inserted within the stator frame, engaging keybars and are stacked together to form the stator core. An end-on view is shown in FIG. 1 of a stator core 10. Since it is assembled, the laminations that make up the core are not separately discernable from this perspective. The core is held to its frame (not shown) by keybars 6, but the core itself is held together by through-bolts 12, which are literally long metal bolts that extend through the length of the core, keeping all of the laminations together.
Both during initial assembly and during maintenance, the stator core needs to be compressed. This is typically accomplished via hydraulic tensioners that are attached to the ends of through-bolts. The hydraulic tensioners press the laminations of the core, while pulling on the through-bolt. This is then repeated several times. Often a tensioner is placed on every through-bolt, though sometimes a strategic placement of tensioners is used at less-than every through-bolt.
Determining how much the core has compressed, however, is still difficult and is usually left to the experience of the technician compressing the core. A knife test, which is attempting to insert a thin knife between laminations, is common practice in the field to test compression. In some cases linear voltage differential transformers (LVDTs) are used. An LVDT can measure movement as little as fractions of an inch or centimeter. The body of an LVDT is typically mounted to a fixed point, while the plunger maintains contact on an object to measure the displacement. The LVDTs are mounted on the keybars or frame, on both sides of the core, to measure how much the core is being compressed. Adjusting measurements to account for deflections in the frame, where the LVDT is mounted, is difficult and reduces the accuracy.
What is needed therefore is a method of measuring the compression in a stator core that is easy to use and accurate.
Other difficulties with the prior art also exist, some of which will be apparent upon further reading.